1. Field of the Invention
The invention relates to a combination bearing including a plurality of rolling bearings stacked with their center lines extending in a vertical direction.
2. Description of Related Art
A rotation shaft of a sprinkler, for example, as illustrated in FIG. 5, is supported by a combination bearing 90 in which a plurality of rolling bearings 91 are vertically stacked. Lubricating oil (oil) Q is stored in a bottom portion of a bearing housing 98. Some rolling bearings 91, 91 (the lower two in the case of FIG. 5) of the plurality of rolling bearings 91 are immersed in the lubricating oil.
Each of the rolling bearings 91 illustrated in FIG. 5 is an angular contact ball bearing. The angular contact ball bearing, upon rotation, exerts an action (pumping action) that causes the lubricating oil to flow from one axial side to another in an annular space 94 between an inner ring 92 and an outer ring 93 (see, for example, Japanese Patent Application Publication No. 2007-263266 (JP 2007-263266 A)). The plurality of rolling bearings 91 are arranged so that the lubricating oil flows in an upward direction due to the pumping action. Thus, when the shaft 99 rotates, the lubricating oil Q stored in the lower portion can be delivered to the upper (third and fourth lowest) rolling bearings 91. As a result, lubricity throughout the combination bearing 90 can be ensured.
When the rolling bearings 91 rotate, the lubricating oil in the annular space 94 moves toward the outer ring 93 due to centrifugal force, and is caused to flow upward along an inner circumferential surface of the outer ring 93 due to the pumping action. Thus, the oil level of the lubricating oil in an inner space 95 between the inner ring 92 and the cage 96 decreases in the second lowest rolling bearing 91, as well as the lowermost rolling bearing 91, both immersed in the lubricating oil while the rolling bearings 91 are not rotating. Instead, air flows into the inner space 95 from upper sides of these rolling bearings 91. The air is more fluid than the lubricating oil. Therefore, if the rotation is continued in this state, the air that has flowed into the inner space 95 in the lowermost rolling bearing 91, for example, moves to the inner circumferential side of the outer ring 93, as indicated by the arrow X in FIG. 5. This causes the air to be mixed into the lubricating oil Q heading upward. If the amount of the air mixed increases, the pumping action weakens, which may prevent the lubricating oil Q from reaching the upper (uppermost or next) rolling bearing 91.